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3 Conclusions and Recommendations CONCLUSION: U.S. participation in Euclid would represent a valuable first step toward meeting one of the science goals (furthering dark energy research) of NWNH for WFIRST. However, Euclid on its own does not provide a viable alternative for achieving the broader NWNH goals for the WFIRST mission, nor does it achieve the more ambitious goals for WFIRST’s dark energy measurements. Euclid will carry out an exciting science program that should yield important scientific results. Benefits of U.S. participation in the Euclid Consortium should also include detailed knowledge of, experience with, and access to Euclid instruments, data pipelines and products, simulations, and modeling, and most importantly, opportunities for U.S. scientists to participate fully in this promising mission. This knowledge could help optimize the science return of the WFIRST mission as well. Such an investment will further the goals of NWNH, be helpful to the preparations for WFIRST, and enhance WFIRST’s chances of success. The committee reached this conclusion that a U.S. role in Euclid would be beneficial to U.S. science, although as NWNH concluded and as the committee heard in evidence presented by several experts, Euclid cannot carry out the full science priorities envisioned in NWNH. WFIRST will uniquely carry out the important NWNH goals of exoplanet studies and a guest observer program.51 Moreover, the presentations to the committee that offered direct comparison of Euclid and WFIRST capabilities for dark energy research showed clear advantages of WFIRST for both the galaxy clustering (baryon acoustic oscillation, BAO) and weak gravitational lensing techniques that make up the Euclid mission. In addition, WFIRST includes the supernova technique whose demonstrated, well-characterized performance serves as an important complement to BAO and weak gravitational lensing. When combined with the results of the Kepler mission, WFIRST’s gravitational microlensing planet search will produce a comprehensive demographic survey running from Mercury- scale orbits to beyond the snow line in hundreds of exo-solar systems, data that will provide an essential observational constraint on the physics of planet formation. The remarkable potential of a deep multiband, near-infrared galaxy survey over a large fraction of the sky and a stellar survey covering the galactic plane and halo will go unrealized without WFIRST. These near-infrared surveys should deliver significant science benefit to multiple areas of astronomy. WFIRST’s observations in Guest Observer mode will, for example, invigorate studies of stellar evolution, providing precise ages for star clusters and an infrared probe of the most vigorous sites of star formation in our Galaxy and its neighbors. RECOMMENDATION: NASA should make a hardware contribution of approximately $20 million to the Euclid mission to enable U.S. participation. This investment should be made in the context of a strong U.S. commitment to move forward with the full implementation of WFIRST in order to fully realize the decadal science priorities of the NWNH report. 11
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RECOMMENDATION: In exchange for this small, but crucial, contribution, NASA should secure through negotiation with the European Space Agency both a U.S. position on the Euclid Science Team with full data access and the inclusion of a team of U.S. scientists in the Euclid Consortium that would be selected by a peer- reviewed process with full data access as well as authorship rights consistent with Euclid policies still to be formulated. As with all NASA missions, the expectation is that NASA will ensure and support access to the Euclid data for the wider U.S. community once the proprietary data period has expired. This involvement is especially important for Euclid, whose survey data should enable significant legacy science. Through its direct involvement in Euclid, the U.S. science team will enhance not only the U.S. capabilities to study dark energy but also the broader use of the Euclid data by the U.S. community. If software developed by the U.S. Euclid team is made available to the community when the Euclid data become public, this step also will enhance the broader community’s ability to use the Euclid data. While the U.S. role on the EST should be more carefully delineated through negotiations with the European Space Agency, it could, for example, involve leadership in Euclid legacy science, defined as providing value-added data products to the community that combine relevant ground and space data sets (including those from WFIRST), given the importance of data dissemination to the U.S. community. NASA and the European Space Agency are discussing several possible options for the U.S. hardware contribution. While the two agencies should determine the nature of the U.S. contribution, contributions with the maximum science benefit to the Euclid mission will benefit both the European and the U.S. communities.52 The reaction wheel could offer a significant improvement in mission efficiency. The filter wheel would also provide some benefit to the mission and would take advantage of U.S. expertise in designing filter wheels (e.g., for the Hubble Space Telescope). The near-infrared detectors, if characterized by U.S. instrumentalists who have expertise in this area, would be beneficial but would likely be the more expensive of the options. The science goals of WFIRST go far beyond those of Euclid, and WFIRST is central to realizing those goals and to maintaining U.S. leadership in astronomy and astrophysics in the next decade and beyond. The committee notes that early expenditures on pre-phase A studies, particularly those that engage the community, often accelerate mission timelines and reduce costs. Mission collaboration costs often grow, as demonstrated by the U.S. participation in the Planck mission discussed above. The committee recommends a tripwire for the hardware costs at 50 percent above the recommended value of $20 million. RECOMMENDATION: NASA should seek independent community review of any financial commitment for hardware expenditures beyond $30 million for Euclid. The committee recognizes that support for this science team will cost an additional ~$2 million per year for about 10 years, for a total that is similar to the hardware investment. That is a necessary expense and not the kind of increase discussed herein. Given the constraints on the NASA budget it is crucial that the contribution to Euclid not grow and impact the rest of the space astrophysics program. The Decadal Survey Implementation Advisory Committee (DSIAC) proposed and described in NWNH would provide an appropriate mechanism for such a review. CONCLUSION: The committee concludes that the combination of data from planned U.S.-led ground-based surveys with Euclid and WFIRST data will enhance the science return from both the ground- and space-based surveys, and that a coordinated, strategic approach to managing these joint data sets could position the U.S. community for a leadership role in their scientific exploitation. 12
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The necessity of ground-based data to Euclid’s scientific success provides a potential basis for negotiating agreements between the ground-based projects and the Euclid mission, which could lead to increased U.S. scientific participation in Euclid. The Department of Energy and the National Science Foundation are the predominant sources of support for two of the U.S.-led ground-based projects, the Dark Energy Survey and the Large Synoptic Survey Telescope. The degree to which these agreements might be coordinated with the proposed NASA participation in Euclid is a matter that the Astronomy and Astrophysics Advisory Committee, which is charged with considering interagency issues, could consider. Obtaining the full scientific benefits of the combined ground- and space-based data will likely require joint processing and analyses of the combined data sets at the pixel level. This deep level of engagement in the data argues for direct collaboration between the ground- and space-based projects. Such joint analyses would require significant data management resources that are not currently budgeted within these projects. 13